This invention relates in general to ultrasonic measurement systems. More specifically, it relates to an ultrasonic densitometer that uses slow torsional waves to measure the density or density related parameters of fluids.
There are many known methods for measuring the density .rho. of fluids. For liquids, the most common is an inexpensive float-type hydrometer, but it is not adequate for rapidly changing dynamic conditions or for gases. More sophisticated densitometers utilize density-dependent physical properties such as electrical conductivity, dielectric constant, or gamma-ray absorption.
Many acoustic densitometers are also known. In general, they utilize the resonant frequency characteristic of a structure such as a U-tube or a circular cylindrical shell, sometimes including a vane across a diametrical plane of the shell. Resonant shell densitometers can be highly accurate, but they are limited in application because they require a large volume of fluid (pipe diameters of 50 mm) for an accurate .rho. determination. They are therefore too large for convenient use in aircraft or other engine applications where the rate of fuel consumption is less than about 1 kg/sec. Likewise they are too large for use with liquids contained in ordinary laboratory test tubes. Resonant tube densitometers also provide high accuracy (about 0.1 mg/cm.sup.3) but they are limited to densities less than 3 g/cm.sup.3, limited in the sampling mode to low flow rates and limited to temperatures close to ambient. Also, response time, despite a small sample volume, typically less than 1 ml, is slow, ranging from 0.5 to 5 minutes.
Heretofore, in a common application, measuring fuel density in aircraft engines where very limited fuel volumes are available, a device using concentric metal tubes whose electrical capacity is proportional to fuel dielectric constant has provided the most accurate measurements. But even these capacitance tubes require a flow channel diameter of at least 20 mm and preferably over 25 mm. Other limitations are that accuracy is reduced for fuels other than JP-4 and JP-5 and if the fuel temperature ranges over too wide an extreme, e.g. from -60.degree. C. to over 100.degree. C. Absorbed moisture in fuels would also degrade the accuracy, due to the disproportionate perturbation of average dielectric constant by a small quantity of water. Conductive or conductively-contaminated liquids cannot be measured by a capacitance type densitometer.
It is also known to use torsional waves in ultrasonic systems. For example, U.S. Pat. No. 2,988,723 to Smith et al discloses a sonic wave conductor to measure the level of a liquid in a tank. The conductor includes a generally cylindrical core and fins secured on its outer surface and extends vertically in the liquid. A measurement is made by detecting an echo generated at the interface of the conductor and the surface of the liquid. The fins amplify what would otherwise be a weak echo. The sound energy can be a torsional wave for some fin designs but it does not measure the density or other characteristics of the liquid. Also in this Smith system the type of sound energy, e.g. torsional or longitudinal, is not particularly significant.
In general, work with torsional waves has used waveguides having a circular cross section. However, it has been known that the velocity of a torsional wave in waveguides formed of an elastic material with a rectangular cross section is reduced by a shape factor K. This observed velocity reduction was not previously related to the density or other density-dependant characteristics of the fluid surrounding the waveguides. Characteristics of fluids have been investigated by ultrasonic wave guides, but using sound energy in vibrational modes other than torsional. For example, a reduction in the velocity of flexural waves in aluminum strips immersed in water has been reported, but there was no association between this effect and the density of the water. Later work with flexural waves demonstrated that there were significant practical difficulties due to dispersive propagation that imposed limits on probe dimensions and bandwidth. Also, depending on design and operating parameters, there can be substantial attenuation of the flexural wave energy due to its radiation into the fluid.
It is therefore a principal object of this invention to provide an ultrasonic densitometer that accurately measures the density of a wide variety of fluids including liquids, gases under high pressure, two-phase liquids plus vapor and hostile liquids in sealed containers.
Another object is to provide an ultrasonic densitometer that operates over a wide temperature range.
A further object is to provide an ultrasonic densitometer that operates rapidly and under dynamic conditions.
Yet another object is to provide an ultrasonic densitometer that measures the density or density related parameters such as mass flow rate, of flowing fluids including flow in conduits with a very small diameter.
A still further object is to provide a densitometer that operates in a pulse-echo mode or a through-transmission mode utilizing narrowband or broadband pulses.
Another object is to provide a densitometer that measures the density of a fluid along a curved path.
Still another object is to provide a densitometer that is acoustically weighted to correspond to the shape of a container or segmented to yield a density profile.
Yet another object is to provide a densitometer that measures one of a variety of parameters such as liquid level, viscosity, condensation or liquid mass in a partly filled container of arbitrary shape.
A further object is to provide a densitometer that can measure the density of a fluid with high resolution and utilizes relatively low cost electronic instrumentation.
Another object is to provide a densitometer that measures fluid density in conjunction with the measurement of at least one other parameter of the fluid.